1. Field of the Invention
The present invention relates generally to laser interferometric methods and apparatus for optical detection of small transient surface motion and particularly for detection of very small transient surface motion of an object subjected to ultrasound, independently of the optical surface quality.
2. Background Art
Interferometry is a well known technique for measuring the phase difference between two or more optical beams. Two-beam interferometers, where one of the optical beams is back reflected by an object surface and the other beam is used as a reference, are used to monitor small deformations on an object or workpiece under test or small displacements of a surface of an object or workpiece under test. Laser ultrasonics can advantageously be used for nondestructive testing in order to measure the thickness of objects or to monitor defects in materials. When industrial applications involve the inspection of an optically rough surface, the ultrasonic information is encoded in a laser beam with speckles.
For detection on optically rough surfaces, the optical sensor must be able to efficiently process the back reflected speckled light. Three different types of interferometers have been previously proposed and developed to efficiently process speckled light for detection of small transient ultrasonic signals: confocal Fabry-Perot interferometers (CFP), adaptive interferometers based on two-wave mixing in photorefractive crystals (TWM), and Multi-channel random-quadrature (MCRQ) interferometers.
With the CFP and TWM interferometers, processing the speckles is carried out optically, and a single photodetector is then used to detect the demodulated signal. With the recently developed MRCQ interferometer, as described in published U.S. patent application US20090027688, processing the multiple speckles is carried out electronically using an array of photodetectors instead of a single photodetector. Each detector element of the array is optimized for single-speckle detection. As shown in FIG. 1A, the MRCQ interferometer 10 comprises a laser source 12 that is adapted to generate a laser beam 14 of a given intensity. The laser beam 14 is split into an object beam 16 and a reference beam 18 using a first beam splitter 20. The object beam 16 is then directed onto a scattering surface of an object 24 subjected to ultrasound using an optical lens 26. The back-scattered light 28 is collected by the lens 26, thus generating a scattered object beam 30. The reference beam 18 is expanded by means of a beam expander 32, and directed by means of mirrors 31, 35. The reference beam 18 and the scattered object beam 30 are then combined using a second beam splitter 33, thus forming two interference beams 34, 36. The two interference beams 34, 36 are each received by two detector arrays 38, 39 of photodetector elements 38i, 39i, respectively, and converted into electrical interference signals, which are processed by a parallel processing circuit 40.
The phases of the portions of the interference beam arriving at the detector elements are random and not correlated with each other due to the speckled interference beam that results from a rough surface of the object whose displacement is measured. Thus, every detector of the detector array 38, 39 receives another speckle pattern with random and non-correlated phases. A processing circuit used with the MRCQ interferometer is also described in U.S. patent application US20090027688 and shown in FIG. 1B. The processing circuit is used to carry out electronic parallel signal processing based on signal rectification, i.e., on the rectification of the amplitude of a sinusoidal signal. The processing circuit is used to generate an output signal proportional to the rectified displacement of the workpiece surface.
However, it has been shown that for measurement values near or below the noise level, the rectification process does not effectively rectify the signals and the output amplitude is lower than the amplitude expected for a response proportional to the absolute value of displacement. Furthermore, for signals with ultrasonic frequencies of more than a few tens of MHz, demodulation based on signal rectification is no longer efficient. With increasing detection bandwidths, the noise amplitude also increases and the signal is often of smaller amplitude.
U.S. published patent application US 20090122322 describes a linear demodulation scheme exhibiting high sensitivity. Linear-demodulation is achieved by using a frequency shift or a Doppler shift in one of the interferometer arms and by synchronizing the detection to this applied frequency/Doppler shift. Practically, the frequency shift may be achieved by mounting a mirror on a piezo electrical translator and applying a ramp signal to the piezo translator, thereby displacing the mirror at a constant velocity for the duration of the ramp.
Although the above described linear detection has shown a good signal-to-noise ratio, it has shown to present some limitations. When using a piezo-mirror, the above described linear detection technique requires excitation with a ramp signal. The detection is thus not continuous because a reset in the ramp is needed. Furthermore, with the technique described in US20090122322, the Doppler frequency must be well below the frequency bandwidth of the ultrasonic signal in order to be efficiently filtered out and to get a clean ultrasonic signal.
A new scheme for linear demodulation is proposed to overcome the above mentioned limitations.